Well, in the beginning the law was not bad. Everbody who did invest in solarpower or windpower has been assured that the electrical energy produced by his power plants will be sold to a fixed price. This price was even over the normal market price, and therefore the investment seemed profitable.
The law made sense in the beginning, because if you look at the graph in the first posting from AndreaDB: Nobody will invest in solar power plants, if you earn 0 Euro with your electricity when the sun is shining at noon.
Also there are some other benefits for renewable energy:
First, their is the fixed price which you have to pay. If the market price drops to 6 cent for a kWh, you still have to pay 19cent.
Second. the distributors of energy have first to buy the renewable energy which is on the market. If there is energy made out of fossiles, they first have to buy all energy produced by solar - and wind- powerplants.
This also makes sense, because this way the renewable energy gets sold. Otherwise nobody would take it because it is to expensive (because of the fixed price).
This both laws did lead to massive investions into green energy in Germany.
But now the law has to be changed. There is too much energy from solarpowerplant which has to be sold at a fixed price. The prices for energy on the market are low as they never have been, but the consumer has to pay more and more for his energy.
They have already tried to lower the fixed price (which is now a huge problem), but well, the lobby was too strong.